To realize a photochemical reactor capable of simultaneously producing photochemical reactions under a plurality of conditions by uniformly and efficiently irradiating the fluid, which flows through a finer flow channel, with light.

The photochemical reactor for irradiating the fluid flowing through the fine flow channel with light to produce the photochemical reactions includes a light source for emitting output light, a light branching means for branching the output light to emit a plurality of branched lights, and a plurality of reaction containers having fine flow channels coated with a catalyst and in which the fluid flows, and uniformly irradiating the whole of the fine flow channels with a plurality of the respective output signals branched by the light branching means.